1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) intended to increase the luminance of a display screen thereof so as to be suitable for an alarm display device to be mounted on a vehicle such as an automobile or the like.
2. Description of the Related Art
FIG. 4 is a sectional side view of a lighting unit and a liquid crystal display device using it described in Japanese Patent Unexamined Publication No. Hei. 9-281492. In this case, a back light unit called an edge light type or a side light type using a light conduction plate 3 is employed as a lighting unit for irradiating a liquid crystal panel 8 of an LCD 1.
A housing 2 having an upper surface portion and a lower surface portion respectively shown in the upper and lower directions in the figure is provided with the light conduction plate 3 made of light transmission material at the center portion thereof. A fluorescent tube (hereinafter referred to as a CCT) 4 of a hot cathode tube or a cold cathode tube in the form of a linear light source is disposed at one end side of the light conduction plate 3. The linear light emitted from the CCT 4 is condensed at the end face of the light conduction plate 3 by means of a reflecting plate 5 having a curved surface and then conducted within the light conduction plate 3. The light from the CCT 4 thus conducted within the light conduction plate 3 is reflected upward by a reflecting sheet 6 disposed at the lower back side of the light conduction plate 3. In this manner, the light from the CCT 4 is formed as a planar light source, and the light from the planar light source transmits a light diffusion plate 7 disposed at the upward portion of the light conduction plate 3 thereby to illuminate the liquid crystal panel 8 provided at the uppermost portion of the housing 2 from the lower direction thereof. The light diffusion plate 7 is formed by an optical part such as a prism sheet or an element formed by translucent material effective for light diffusion.
Since the light from the CCT 4 transmits the light diffusion plate 7, the light is formed as a planar light source with a uniform luminance and then illuminated on the entire surface of the liquid crystal panel 8. As a result, the LCD 1 is formed as a dot matrix type LCD which displays characters or images on the display screen of the liquid crystal panel 8 in the form of the aggregation of dots, for example.
FIG. 5 shows another conventional example of the LCD having such an edge light type back light unit.
In this case, a plurality of CCTs 4 are disposed at the lower portion of the housing 2. A part of the light emitted from the respective CCTs 4 is reflected upward by a reflecting sheet 5 which is formed in a wave shape and disposed beneath the CCTs. The reflected light passes through a light modulation film 9 and transmits the light diffusion plate 7, whereby the light is formed as planar light with uniform luminance and then illuminated on the entire surface of the liquid crystal panel 8 from the lower direction thereof.
In each of the conventional LCDs shown in FIGS. 4 and 5, the back light unit serving as a planar light source still has the following problem to be improved.
The luminance of the light emitted from the surface of the side light type CCT 4 is usually about 30,000 cd/m.sup.2. The light emitted from the CCT 4 as the linear light source passes through the light conduction plate 3 and the light diffusion plate 7 or the light modulation film 9, so that the luminance of the light is attenuated. Thus, the luminance of the light illuminated on the liquid crystal panel 8 from the planar light source is reduced. Further, the luminance of the displayed image is reduced to about 150cd/m.sup.2 due to the transmittance of the liquid crystal panel 8.
In the case of displaying various kinds of alarms within a vehicle such as an automobile, the luminance of the displayed image is desirably 200 cd/m.sup.2 or more at the maximum. However, the luminance of the displayed image of the LCD is generally about 150 cd/m.sup.2 at the maximum. In the aforesaid conventional examples, since the luminance of the displayed image of the LCD is reduced to about 150 cd/m.sup.2 due to the attenuation, the shortage of the luminance of the displayed image arises. Therefore, the aforesaid conventional LCDs are disadvantageous in that the visibility and the attention calling function thereof are deteriorated when used for a particular object such as a vehicle-mounted alarm display.